The Hrb genes encode the Drosophila homologs of the vertebrate A-B hnRNP proteins. These proteins can be isolated from nuclear extracts of Drosophila tissue culture cells as a 40-80S RNP complex. Results from a variety of biochemical experiments indicate that these complexes contain premessenger RNA and that the Hrb proteins are single-stranded nucleic acid binding proteins. Despite the fact that the RNA binding domains of the Hrb proteins show only 60% identity to that of the A-B hnRNP proteins, they are very similar in nucleic acid binding properties, RNP complex formation, and basic core protein complexity. Genetic analyses of both Hrb genes are in progress. Using a small deficiency that removes the Hrb98DE gene and several others, we have isolated 5-6 lethal complementation groups, which are being tested to determine which represents Hrb98DE. For Hrb87F, we have identified a fly strain that has a P element construct inserted in the 5' untranslated region of the gene. These flies show somewhat reduced viability and dramatically reduced fertility. They do produce some Hrb87F transcripts of approximately normal size, probably by splicing out the insert. Interestingly, flies with reduced levels of Hrb87F transcripts produce increased levels of Hrb98DE transcripts, suggesting the existence of a feedback mechanism to ensure adequate levels of hnRNP proteins. The R16 cDNA clone encodes a protein with two putative RNA-binding domains and a proline-rich C-terminal region. The 5-6 different transcripts produced from the gene show a complex pattern of developmental regulation. Genomic clones and cDNA clones corresponding to the different transcripts are being characterized.